Magnetocardiogram (MCG) was recorded for normal subjects and patients with right or left ventricular hypertrophy using a second derivative superconducting interference device (SQUID) gradiometer and the results were compared with those of electrocardiogram (ECG). MCG with a second derivative gradiometer localized current sources more easily than did ECG. Therefore, the waveforms of the MCG varied more than ECG dependent on recording sites. The current source near the anterior chest wall was detected readily, as this gradiometer measures the spatial differentiation of the magnetic field. In normal subjects, dipoles located on the right and directed to the right and superiorly, apparently of right ventricular origin, appeared earlier and were more clear than with the ECG. In addition, in patients with right ventricular overloading, MCG reflected an increased force to the right more clearly. In patients with left ventricular hypertrophy, MCG showed the T wave inversion more frequently than did ECG. The electromotive forces deduced from isomagnetic and vector arrow maps were in good agreement with the excitation sequences of previous reports. MCG could detect multiple dipoles by these maps, especially opposing dipoles which were difficult to detect by ECG or isopotential maps. These results suggested that MCG provides information different from ECG. Thus, the use of MCG in addition to ECG might be helpful in determining the current sources more accurately.